Electrochemistry Unit 4 Material

Potentiometry

• a way to measure cell potential (Ecell) without drawing appreciable current

• 2 electrode set up WE and RE

• Current is 0 (does not impact voltage measurements)

• Based on the electrode, the way concentration is observed changes

  • Metal electrode might meausure voltage at redox equilibrium (NO CURRENT)

  • Ion-selective electrodes create a voltage because one ion is unevenly distributed/interacting across a selective membrane. (NO CURRENT)

Ion-selective electrode

• responds to one specific ion in solution

• electrode develops a voltage depending on how much of that specific ion is present

• Relate that voltage to ion concentration using Nernst equation

Amperometry

• measures current at constant applied potential

• 3 electrode set up

• more analyte = more reduction = more current

Voltammetry

• Measures current (I) vs. Applied Potential (E)

• aka vary voltage and measure current

• 3 electrode set up

Between what two electrodes is cell potential measured?

• WE and RE

equation sheet

• E = potential of indicator

• To get E cell subtract the value of the reference electrode

• E standard cathode is the one with circle

Auxiliary Electrode / Counter Electrode

• When ox/red happens at the WE, electrons must go somewhere or come from somewhere

• If WE is oxidizing, the CE will reduce to keep charge balance

• Makes sure current flows through CE and nothing else; this keeps RE stable so that voltage control is accurate

• Carries current required for WE run

Galvanic Cell

• spontaneous

Electrolytic

• non-spontaneous

• must apply current to drive rxn

Current vs Potential Graph (Voltammetry)

• positive current = anodic = from oxidation (loss of e- at WE)

• negative current = cathodic = from reduction (gain of e- at WE)

Which electrode should be non-polarizable?

• RE

CE polarizability

• easily changed with small amounts of current flow

WE, RE, and CE Positioning in a Electrolysis cell

• WAR clockwise

Potentiostat Role

Allows you to control the applied potential and measure current as potential changes.

Why is WE close to RE?

• to minimize iR drop → voltage drop caused by solution resistance

• less distance apart means less solution that can undergo resistance

Ohmic Potential (iR drop)

• solution between electrodes has resistance (R)

• When current (i) flows through, some voltage is lost in the amount V = iR

Overpotential (n)

• extra voltage needed to make electrode reactions occur at a practical rate

Equation

Eapp = Ecat - Ean - iR - n

How to reduce iR drop

• add supporting electrolyte → more ions in solution means lower resistance

• use lower current

• use a more conductive solvent

Overpotential (n)

• Extra voltage needed to overcome activation energy or reaction of an electrode

• make a reaction fast enough

• depends on the electrode surface (ag, pt, etc)

• some electrode surfaces lower the activation barrier better than others

Common Reference Electrodes

• Saturated Colomel Electrode (SCE) → based on HgCl2 and saturated with KCl

• Ag/AgCl Reference Electrode → based on AgCl and saturated with KCl; lower E standard cell than SCE

What makes a good RE?

1. Reversible reaction: the electrode reaction can go forward/backward easily, so the potential is predictable.

2. Little hysteresis: Electrode returns to the same voltage

3. Follows Nernst equation: its voltage changes in a predictable way based on ion concentration.

4. Stable potential over time: the voltage does not drift while you are measuring.

5. Constant ion concentration: in Ag/AgCl or SCE, [Cl−][Cl−] affects voltage, so saturated KCl keeps [Cl−][Cl−]constant. Constant [Cl−][Cl−] = constant reference voltage.

Do working electrodes have ranges?

yes

What happens if you go outside the range?

• water may oxidize/reduce

• gas evolution (H₂ or O₂)

• electrode surface changes/fouls

• background current rises

• bad data

How to choose material?

• based on the potential needed for the analyte reaction

Other considerations for a three electrode set-up:

1. Need a supporting electrolyte (alkali metal salt that doesn’t react w/ electrode)

2. Had conductivity

3. minimizes iR drop

4. decreases migration → want measured current to be only from diffusion

5. Need to deoxygenate with N2 bc oxygen can be reduced

What process causes charging of the double layer

Nonfaradic

Non-Faradic Charging

• Current from charging/discharging the electrical double layer at the electrode surface.

• No rxn

Faradic Charging

• Current from an redox reaction where electrons transfer between electrode and analyte.

Nonfaradic

• voltage applied

• metal gains charge

• opposite charge ions line up in solution (+)

• creates electrical double layer

Faradic

• Faradic current occurs due to redox reaction at the electrode

Non - faradic current on a graph

• no peaks

• distance between forward and backward scan is the double layer capacitance

Faradic Current on graph

• forward scan peak - red/ox occurs at a strong potential

• backward scan peak - opposite rxn

What is total current

i (faradic) + i (nonfaradic)

Mechanisms of Mass Transport

1. Migration (movement of charged ions due to magnetic field)

2. Diffusion (high to low concentration)

3. Convection (stirring)

What is electrode polarization

• need for extra voltage because system cannot keep up perfectly

Concentration Polarization

• reactant at surface of electrode gets depleted

• electrode consumes Ox faster than it can diffuse to the electrode

What is current limited by

• mass transport

Adsorption/desorption polarization

1. molecule may need to stick to electrode for reaction to proceed

2. if adsorption is slow, current is limited

Charge/Transfer Polarization

• electron transfer itself is slow (activation energy)

Linear Sweep Voltammetry (LSV) Waveform

• if negative sweep, sweep starts at a more positive potential and moves steadily toward a more negative potential.

• more negative potential makes the WE better at reducing

• More positive Ecell is reduced first

What gets reduced first? more positive or less positive E1/2

more positive

LSV Voltamogram with Stirring

• no current because not much reduction is happening

• more negative voltage increases reduction which increases current

• current is proportional to amount of reduction happening

• plateau → limited by mass transport (rate limited by diffusion)

Estand. is approx equal to half-wave potential

what is iL proportional to

C

with rotating disk:

• diffusion layer thickness is constant

• current & flow is stable